1. Field of the Invention
The present invention generally relates to methods and systems for preventing collisions between aircraft and terrain. More particularly, the present invention defines terrain warning systems and methods that inform a pilot to avoid collision with terrain and when hazardous terrain has been traversed.
2. Related Art
Ground Proximity Warning Systems (GPWS), Ground Collision Avoidance Systems (GCAS) and/or Terrain Awareness and Warning Systems (TAWS), are used in aircraft to inform pilots or other flight crewmembers of likely or imminent collision with terrain. For simplicity, these and other system for warning pilots of potential collision with terrain are collectively and individually referred to herein as TAWS. TSO-C151a (Technical Standard Order), which is fully incorporated herein by reference including its Appendices, and RTCA DO-161A, prescribe minimum operational performance standards for TAWS equipment in the United States.
TAWS includes a Forward Looking Terrain Avoidance (FLTA) function which looks ahead of an aircraft along and below the aircraft's lateral and vertical flight path and provides alerts to a flight crew if a potential Controlled Flight Into Terrain (CFIT) threat exists. CFIT is an aircraft accident where a completely airworthy aircraft is inadvertently flown into terrain (which may include man-made obstacles) or water. In the United States, Class A TAWS equipment must provide terrain information on a display system such as a weather radar display or other video output device. In such a system, visual and aural signals are provided to a flight crew for both caution and warning alerts in one or more of the following situations:                (1) excessive rate of descent;        (2) excessive closure rate to terrain;        (3) negative climb rates/loss of altitude after take-off;        (4) flight into terrain when not in landing configuration;        (5) excessive downward deviation from an ILS (Instrument Landing System) glideslope;        (6) an FLTA function that looks ahead of the aircraft along and below the lateral and vertical flight path and provides suitable alerts if a potential CFIT exists; and        (7) a Premature Descent Alert (PDA) function that determines if an airplane is hazardously below the normal (typically three degree) approach path for the nearest runway.        
A “caution alert” as used herein is provided when a potentially hazardous flight condition is encountered, immediate crew/pilot attention is required, and a potential for crew/pilot action exists. In a TAWS, a typically caution alert is in the form of an audible notification of a flight condition, e.g., “terrain ahead” and/or corresponding visual indicators of the flight condition such as an amber or yellow light on a display. A “warning alert” as used herein is provided when a potentially hazardous flight condition is encountered and immediate crew/pilot action is required to avoid a hazardous flight condition. Typically, a warning alert may be in the form of an audible: instruction to take action (e.g., “terrain, terrain; pull up, pull up”) and/or corresponding visual indicator (e.g., a red light or symbol on a cockpit display) notifying a crew or pilot that action is required to avoid an imminent threat. In most instances a warning alert is only provided after a caution alert is provided. The differences between these alert types are made primarily to distinguish between severities of threats from terrain that one may encounter during flight.
Forward Looking Terrain Avoidance (FLTA) in TAWS uses a “look ahead” or “sensor” profile for determining when to generate terrain caution and/or warning alerts. These sensor profiles typically include a projection of the aircraft's future position using two or more segments including a response segment, which projects the flight path of the aircraft a certain length of time in front of an aircraft, and a climb profile, which projects an angle of increasing elevation that the aircraft might take to avoid terrain. In a spatial sense, the climb profile is connected at the forward end of the response segment. This look ahead profile is compared with terrain data to determine whether an aircraft is encountering a terrain threat. If terrain data intersects the TAWS look ahead profile, then a caution or warning alert is given to the flight crew depending on the severity of the threat. The primary difference between generation of a caution alert and a warning alert is the length of the response segment in a look ahead profile. That is, an increased length of time is used for a caution alert, whereas a smaller length of time for response is used for a warning alert.
The climb profile is a prediction of the change in aircraft altitude for overcoming terrain threats and is based, at least in part, on an aircraft climb angle. A “climb angle” is the angle of a flight path measured from the horizon.
Fixed profiles used in some conventional FLTA processing may not take into account the actual climb angle obtainable by a particular aircraft, but rather utilize a fixed, conservative climb angle that is presumed to be safe for all types of aircraft that may utilize a TAWS. Accordingly, some conventional sensor profiles are static by nature and do not account for certain differences between types of aircraft or take into account actual performance characteristics of the aircraft in which the TAWS is present. Variances between, for example, thrust or aerodynamic characteristics and performance of aircraft can significantly alter the true climb angle that may be utilized by an aircraft for escaping a terrain collision threat. Consequently the true climb angle of a particular aircraft may not match the predicted climb profile used with conventional FLTA.
The conventional approach, using a fixed climb profile, has a drawback that the climb profile may not be suitable for all aircraft. For example, if an aircraft is unable to attain a climb angle predicted in the look ahead profile, then the aircraft may not be able to sufficiently avoid a terrain threat when a warning alert is sounded. Alternatively, if the climb profile is too conservative for the actual climb angle obtainable by an aircraft, alerts may be generated before any respective caution or warning alert is necessary or desired. These unnecessary alerts are referred to as “nuisance alerts,” and are generally desired to be minimized. For the foregoing reasons, it is desirable to provide a terrain avoidance profile that considers the specifications and recently demonstrated characteristics of the aircraft for which it is used.
Referring now to FIG. 1, an example of conventional TAWS functionality will be described. The Forward Looking Terrain Avoidance (FLTA) function of the TAWS in aircraft 10 compares numerical representations of look ahead profile 15 (also referred to herein as a “terrain avoidance profile”) with numerical representations of terrain features, for example, mountain 50, stored in a database. For the sake of simplicity, the FLTA described herein relates to the terrain avoidance profiles in association with generating a warning alert. However, profile 15, and the inventive profiles discussed hereafter, may be used for either a caution or warning alert. Notwithstanding, when profile 15 of aircraft 10 at position (A) encounters terrain feature 50, the TAWS in aircraft 10 issues a warning alert including aural alert 25 such as “Terrain, pull up, pull up.” The pilot of aircraft 10 reacts to this warning by following the recommendation of aural alert 25 through positions (B) and (C) until profile 15 no longer conflicts with terrain feature 50. At position (D), aural alert 25 ceases and the pilot may stop increasing the elevation of aircraft 10 to avoid terrain 50, or “level off.”
This conventional approach to TAWS suffers from one or more of the following problems. When visual inspection by a pilot is impossible or impaired, e.g., when flying in fog, darkness, or other visual impairment condition, a pilot of aircraft 10 may be entirely surprised by a terrain warning alert. Since the terrain warning alert provides no indicia of how fast to “pull up” or how much to “pull up,” the pilot will most likely enact the most rigorous increase in elevation possible. Moreover, because aural alert 25 continues to sound until aircraft 10 reaches an elevation safe enough to clear terrain 50, the pilot will continue the most rigorous increase in elevation possible until the terrain is cleared. This type of extreme avoidance maneuver can: (i) induce aircraft 10 to stall, thereby increasing the chance of collision with the terrain, (ii) lead to injury of passengers, flight crew or cargo or at least an uncomfortable ride due to severe course alterations, and/or (iii) cause unnecessary stress on the pilot and/or damage to aircraft 10. With the conventional methods and systems, a pilot may not realize danger has subsided until the aural alert ceases, even though in reality, a timely response by the pilot and a slight or gradual increase in altitude might avoid collision.